On Sunday, the GOP presidential hopeful acknowledged that his position has changed since 2013, when he helped craft a comprehensive immigration reform bill that included a pathway to citizenship for millions of undocumented immigrants.

But that shift, he argued, was a concession to the political realities in Washington –– where House Republicans have refused to consider the issue –– as well as a reaction to recent terrorist attacks he says have exposed holes in the enforcement of existing law.

"It is very clear now more than ever that we are not going to be able to do anything on people that are illegally until we first prove to people that illegal immigration is under control and America is safe," he said in an interview on NBC's "Meet the Press" program. "And ISIS poses a very unique threat unlike anything we have faced in the past."

"The fact of the matter is that Ted has shown a propensity throughout his career in the U.S. Senate to take one position in front of one audience and then change his position in front of another," Rubio said. "He used to be in favor of legalizing people that are here illegally, and he said so in front of one audience. But then he portrays this sort of notion that he's the harshest and hardest when it comes to that issue."

Rubio took sharp criticism from the right in 2013 when he helped craft a sweeping immigration reform bill that included eventual citizenship for undocumented immigrants. Those attacks have resurfaced on the campaign trail, forcing the Cuban-American Rubio to switch course and largely disavow the legislation he'd sponsored.

Rubio defended that switch on Sunday, saying the political realities that have stalled comprehensive reform have caused him to rethink his strategy.

"If circumstances change or you learn something along the way, it's reasonable to say, 'Maybe a different approach will work better,'" he said. "So, for example, on immigration it is clear no comprehensive solution to immigration is going to pass."

Rubio said he's still in favor of legalizing millions of undocumented immigrants, though he emphasized that convicted criminals should be weeded out and denied those rights. He suggested those calling for the immediate deportation of anyone in the country illegally are delusional.